1. Field of the Invention
This invention relates generally to optically generating a pattern of figures, such as an array of spots or an array of scan lines. More particularly, this invention relates to generating such patterns using multi-faceted rotating optical elements.
2. Description of the Related Art
The optical generation of a pattern of spots or scan lines is used in a variety of applications. Digital copiers, printers, fingerprint identification, hand-held bar code scanners, industrial applications, light show entertainment, displays, telecommunications switching and medical applications are a few examples. Perhaps the most common mechanisms for generating patterns of figures are tilting mirrors (e.g., oscillating mirrors driven by galvanometers) and reflections from rotating polygons.
However, optical pattern generators based on tilting mirrors typically have characteristics that make them unsuitable for certain applications. For example, scanning in these systems is typically achieved by tilting a mirror back and forth. But back and forth motion requires that the mirror come to a stop and then reverse direction. This takes time, which limits the scan rate. In order to increase the scan rate of these systems, the mirror often is driven with an oscillating motion at a rate that is near its resonant frequency. However, this severely restricts the patterns that can be generated. For example, it is difficult to generate irregular patterns since the mirror motion is constrained to be oscillatory. The near-resonance condition also limits the range of scan rates that can be achieved. For example, it is difficult to tune such a system over a wide range of scan rates since the near-resonance condition cannot be met over a wide range. If a two-dimensional pattern is desired (e.g., a series of parallel scan lines or a two-dimensional pattern of spots), then typically either a single mirror is tilted in two directions simultaneously or two coordinated, tilting mirrors are used. In many cases the efficiency of the utilization of light, such as laser light, is also important. The efficiency may be defined as the fraction of energy deposited in a desired pattern on the treatment surface compared to the total energy produced by the light source in a given period of time. If a pattern is sparse compared to the background, it is preferable to turn off the light source and scan quickly over the background, and then turn it back on when the light beam has settled over the spot to be exposed and expose the spots in the pattern in such a manner that the light source is efficiently utilized in time. This requires an even more responsive device that can accelerate, decelerate and settle quickly. As a result of these characteristics, galvanometer-based systems are not well suited for high speed pattern generation, particularly if the pattern is an irregular or a sparse one.
In the rotating polygon approach, the sides of a three-dimensional polygon are mirrored and the polygon is rotated about a center axis. As each mirrored side rotates through an incident optical beam, the optical beam is reflected to generate a point on a scan line. The rotation of each mirrored side through the optical beam produces one scan line. If all of the mirrored sides are the same (e.g., make the same pyramid angle with the base of the polygon), then the same scan line is traced over and over. If the mirrored sides are different, then different scan lines can be traced as each side rotates through the optical beam. For example, by varying the pyramid angle of each side, the reflected optical beam can trace a series of scan lines.
However, the rotating polygon approach also has drawbacks that make it unsuitable for certain applications. For example, systems that produce a series of scar lines can suffer from aberrations due to the rotation. In order to trace a series of scan lines, each side has a different pyramid angle that offsets the basic scan line in a direction that is perpendicular to the scan direction. However, as each side rotates through the optical beam, the orientation of the angled side is also rotated. This can cause changes in the amount of offset and/or other unwanted aberrations. One example is scan line bow. The ideal scan line is generally a straight line segment but the actual scan line is often an arc segment. The sag of the arc segment is the bow. In the case of rotating polygon scanners, sides that have non-zero pyramid angles generate bowed scan lines. The amount of bowing depends on the pyramid angle. In a polygon scanner where different pyramid angles are used to trace multiple scan lines or to generate spots at different locations, not only will each scan line be bowed, but the bow will vary from one scan line to the next. The difference between the bow of the top-most scan line and the bottom-most scan line can be significant.
Scan line bow and other effects caused by rotation can cause additional problems, depending on the application. For example, in some applications, the scanning action is used to compensate for motion of the scanner relative to a target so that the optical beam ideally remains at a fixed spot on the target even though the scanner is moving relative to the target. In this case, scan line bow will cause the optical beam to move in the direction perpendicular to the scan direction. If this motion is slow compared to the dwell time of the optical beam on the target, then the bow effectively introduces an unwanted motion in the perpendicular direction. If the motion is fast relative to the dwell time of the optical beam on the target, then the bow, which is a radial deflection, when combined with the uncompensated tangential motion, effectively blurs the optical beam, increasing the spot size of the beam on the target. Typically, neither effect is desirable.
Thus, there is a need for optical pattern generators than can operate at high speeds, particularly for the generation of irregular patterns. There is also a need for pattern generators with reduced aberrations and/or blurring.